Cómo iniciarme en un trío con mi esposa

I previously proposed it to him, but he always refused. A few days ago, he said he's willing to think about it, we're 33 and she's 30, I'm not looking for an extremely experienced guy or woman, rather someone who is also starting out, seriousness, discretion, and above all, friendly good vibes, nothing extreme like that jaja. Also, advice on how to select the right person, thanks